Lightning arrestor device for high power electric installations

ABSTRACT

A lightning arrestor device for protecting an installation ( 3 ) powered by an electric transmission line ( 1 ) includes at least two assemblies connected in parallel between the line terminals, each including a disconnector (P, P′) mounted in series with at least a voltage-dependent variable resistance component. The variable resistance component is a Zener diode, at least one assembly consisting of at least two Zener diodes (Z 1 , Z 2 ) arranged in parallel.

BACKGROUND OF THE INVENTION

The present invention relates to a lightning arrestor device of the typeused in particular for ensuring protection of high power electricinstallations.

DESCRIPTION OF THE RELATED ART

It is known that, in order to ensure protection of such installationsagainst lightning, lightning arrestor devices are employed which areconnected between a mains wire and earth, and which comprise componentswhich, in normal functioning, behave like neutral elements but which, inthe event of violent excess voltages, become conducting, so that theydeviate the current due to the lightning towards earth, thus protectingthe installation disposed downstream against the destructive effects ofthe excess voltage.

As a safety measure standard IEC 61643-1 renders it compulsory toassociate with the lightning arrestor a disconnector whose role is tocut the circuit if the lightning arrestor is destroyed. Suchdisconnectors may be essentially disposed in two configurations.

In a first configuration, the disconnector is disposed, as shown in FIG.1, on the line itself. Under these conditions, it will be understoodthat its disconnection brings about cut of electrical power of theinstallations located downstream.

In the second configuration, shown in FIG. 2, the disconnector isdisposed on the by-pass branch going to earth, so that its disconnectionindeed respects a continuity of service since the current can continueto circulate in the principal branch, but it no longer ensures acontinuity of the protection since it interrupts the by-pass towardsearth.

It will be understood that neither of these two configurations istotally satisfactory, since, after destruction of the lightning arrestorand the activation of the disconnector which is associated therewith,the first has the effect of paralyzing the installation and the secondhas the effect of placing the latter in danger.

It is also known that another quality of a lightning arrestor is itscapacity to allow the excess voltages to pass without any wearoccurring. This is why it appears advantageous to constitute such alightning arrestor from elements which are particularly resistant towear and tear such as semi-conductor junctions of Zener type which,furthermore, present other qualities with respect to the other lightningarrestor devices of the prior state of the art, such as in particulardischargers and varistors.

DE-4124321-A describes a device for protection against excess voltages,comprising components with variable resistance as a function of thevoltage, such as varistors, connected respectively in series withdisconnectors, such as fuses, in at least two circuit branches extendingin parallel between an electrical power line having to be protectedagainst excess voltages and earth.

SUMMARY OF THE INVENTION

The present invention has for its object to overcome the drawbacksmentioned above by proposing means for protection against the effects oflightning which ensure, after a lightning stroke, both the continuity ofservice for the installation and its protection against a subsequentlightning stroke, and this with components of Zener type.

The present invention thus has for its object a lightning arrestordevice for protecting an installation powered by an electrictransmission line, consisting of at least two assemblies connected inparallel between the line terminals and each comprising a disconnectormounted in series with at least one voltage-dependent variableresistance component, characterized in that the variable resistancecomponent is a Zener diode, at least one assembly consisting of at leasttwo Zener diodes arranged in parallel.

According to the invention, the threshold voltages of the respectiveZener type diodes of said assemblies will preferably be adjacent.

In an embodiment of the invention, one of the assemblies comprises aZener type diode of which the threshold voltage is greater than that ofthe Zener diodes of the other assemblies.

The threshold voltage of said assembly is preferably greater than thethreshold voltage of the other assembly by a value equal tosubstantially the voltage difference between the low current and highcurrent values of the other assembly.

As will be set forth hereinbelow, such a differentiation of these diodesmakes it possible to avoid, in cases of very particular characteristicsof excess voltages due to lightning, that the two Zener diodessimultaneously become conducting and be destroyed, in that case leavingthe installation bereft of protection.

In order to make a selection, it is also possible to give one of theassemblies short links and the other assembly long links.

In a preferred embodiment of the invention, at least one of theassemblies constituted by at least two Zener diodes in parallel presentselectrical links between these diodes and the installation which areshorter links than those of the other assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

A form of embodiment of the present invention will be describedhereinafter by way of non-limiting example, with reference to theaccompanying drawing, in which:

FIG. 1 schematically shows a protection assembly according to the priorstate of the art in a configuration excluding a continuity of service.

FIG. 2 schematically shows a protection assembly according to the priorstate of the art in a configuration excluding a continuity of service.

FIG. 3 is a graph representing the characteristic of a Zener diode ofthe type able to be used according to the present invention, namely thevariation of the voltage as a function of the intensity thereof.

FIG. 3a is a schematic graph of the characteristic of a Zener diode,intended to demonstrate the voltage difference ΔV at the terminalthereof, in the case of weak currents and of strong currents.

FIG. 4 is a schematic view of an example of an assembly according to theinvention for protection against lightning.

FIG. 5 is a graph representing the variation of the voltage V as afunction of the variation of instantaneous current di/dt respectivelyfor two Zener diodes in parallel of adjacent threshold voltages, for aZener diode alone of the same threshold voltage, and for a Zener diodeof slightly greater threshold voltage.

FIGS. 6 and 7 are schematic views of two examples of embodiment of anassembly according to the invention for protection against lightning.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a line 1 for electrical supply of an installation 3 whichit is desired to protect against the destructive effects of lightning.To that end, according to the prior state of the art, a lightningarrestor 7 has been arranged between the line 1 and earth 5, i.e. adevice, which, in normal functioning, behaves like a neutral element,i.e. it does not conduct current, but which, in the event of a violentexcess voltage, becomes conducting. In a first configuration ofprotection shown in FIG. 1, there is disposed on the line 1 upstream ofthe lightning arrestor 7 a disconnector device 9, i.e. a device whichhas the effect of opening the circuit when it is traversed by aconsiderable current. In such an embodiment, when an excess voltage, tooviolent due to the lightning, occurs, causing the lightning arrestor tomelt, the latter short-circuits and diverts the current towards earth 5,thus protecting the installation 3. During this action, the disconnector9 opens, thus depriving the installation 3 of any electrical supply.

In the second configuration according to the prior state of the artshown in FIG. 2, the disconnector 9 is placed on the branch conduitupstream of the lightning arrestor 7. Under these conditions, when aconsiderable excess voltage provoked by lightning occurs, the lightningarrestor 7 passes to the state of short circuit, thus diverting theexcess voltage towards earth 5, then the disconnector 9 opens thecircuit. Under these conditions, if the installation 3 remains suppliedwith current after the effect of lightning, it is then noted that it isno longer protected against a subsequent effect, insofar as thedisconnector 9 has opened the branch conduit.

The basic principle of the lightning arrestor device according to thepresent invention consists in exploiting an observation made byApplicant whereby, in dynamics, the residual voltage of a Zener diodevaries considerably as a function of the variation of the current di/dt.

In effect, if two Zener diodes whose threshold voltage is very close,are associated in parallel, with the aid of short connections, and ifthere is plotted on a diagram the variation of the voltage at theirterminals as a function of the instantaneous variation of the intensitydi/dt (curve a), it is ascertained that the course of the curves is suchas shown in FIG. 5. The same has been done with one only of the twoZener diodes, for example the diode, and the corresponding values areplotted on the same FIG. 5 (curve b).

It is noted in this Figure that the characteristic curve of the twoZeners mounted in parallel lies beneath the corresponding curve relativeto a single Zener diode.

Furthermore, the same Figure shows a third curve (curve c) relative to athird Zener diode, of which the threshold voltage is slightly greaterthan the threshold voltage of the preceding two Zener diodes, preferablyby a value ΔV equal to the voltage difference existing between the lowcurrent value Va and the high current value Vb of this Zener diode (FIG.3a).

The curves obtained show that the association of the two Zener diodesfacilitates the insulation of a Zener group, and the selection ordifferentiation in the event of a considerable excess voltage of thisparticular group.

It is known that the placing in parallel of semi-conductorjunctions ofZener type raises problems by reason of the course of the characteristiccurve of the Voltage/Current function f(U/I), of which an example isreproduced in FIG. 3.

In effect, although the voltage is the same at the terminals of twoZener diodes disposed in parallel, it is observed, due to the slightinclination of the curve f(U/I), that the currents which traverse saiddiodes may nonetheless be very different. In this way, on the curveshown in FIG. 3, it is ascertained that, for the same voltage V₁, therespective currents traversing Zener diodes D₁ and D₂ of which thethreshold voltages are adjacent, are traversed by respective currents I₁and I₂ of very different intensities. Under these conditions, it will beunderstood that it will be necessary to match them by a strict sortingfrom their threshold voltage, if it is desired that they distribute thedischarge current equitably.

According to the invention, as shown in FIG. 4, a module has beenproduced, formed by two assemblies E, E′ constituted by Zener diodes Z₁and Z₂ respectively for assembly E and Z′ for assembly E′, which havebeen chosen by sorting so that their threshold voltages are adjacent,with which two respective disconnectors P, P′ have been placed inseries, so that the two inputs of these disconnectors P, P′, are joinedby a short electrical connection 10 which is joined to the line 1, inthe same way as their respective outputs are also joined, by a shortelectrical connection 10′, itself connected to earth 5.

Under these conditions, it is understood that, when a considerableexcess voltage due to lightning occurs, the group of two Zener diodesZ₁, Z₂ begins to conduct first, so that, at the end of life (meltingprovoking a conductivity) of one of the two, the latter short circuitsall the others and it alone supports all the intensity of the currentresulting from this considerable excess voltage. During the passage ofthis high intensity, the disconnector associated with the Zener dioderendered conducting is disconnected, in that case interrupting thepassage therethough.

Under these conditions, it will be understood that the service procuredby line 1 has not been interrupted by the considerable excess voltagetraversing the lighting arrestor and that one of the two, namely the onewhich has not passed to the definitive state of conduction, remainsoperational.

In such an assembly, it is not possible to know that the disconnectionhas taken place. To know this, it is, of course, possible to add to thedisconnector which has passed from the conducting state to thenon-conducting state, annexed means 12 indicating that the lightningarrestor associated therewith is to be replaced.

A “led” diode may thus be associated with each assembly E, E′, whichlights up when the disconnector device 9 is disconnected.

However, it has been ascertained that, by reason of the particularlycomplex nature of the excess voltages due to the lightning, it mayhappen that two lightning arrestors in parallel conduct simultaneouslyand that, consequently, they are simultaneously destroyed, in that caseleaving the installation without protection, the annexed means 12indicating the disconnection.

According to the invention, means will therefore be proposed, enablingthe two assemblies to be differentiated so that one certainly reactsbefore the other.

A first means will therefore consist in differentiating the respectivethreshold voltages of the assemblies E and E′. The threshold voltage ofthe assembly E′ will preferably be higher than the threshold voltage ofthe other assembly E by a value ΔV equal to the voltage differenceexisting between the low current value V_(a) and the high current valueV_(b) of the other assembly (FIG. 3a). This choice makes it possible toconserve a good level of protection at the second stage (namely the oneof which the threshold is higher).

A second means for ensuring differentiation of the Zener diodes willconsist in playing on the reaction times thereof by acting on the lengthof the links of these Zener diodes with the installation.

As shown in FIG. 7, a first assembly constituted by a Zener diode Z₁ andby a disconnector P₁ is connected to the installation by a short link,while a second assembly constituted by a Zener diode Z′₁ and by adisconnector P′₁ is connected to the installation by a long link.

In such a configuration, it has been ascertained that the assembly whichwas “activated” first by a considerable excess voltage due to thelightning was always the one with short link. An additional means isthus available for ensuring the differentiation of the activation of thetwo assemblies.

It is, of course, possible according to the invention, as shown in FIG.6, to constitute two assemblies E and E′ of which one will comprise anumber n of Zener diodes, Z₁, Z₂. . . Z_(n).

The present form of embodiment of the invention is interesting in thatit makes it possible to determine that of the two lightning arrestorassemblies which will certainly be activated in the event of an excessvoltage provoked by lightning.

Although the present invention is usable and presents numerousapplications in the field of direct current, it is, of course, alsousable in the field of alternating current. In the latter case, it goeswithout saying that, instead of a single Zener diode, two Zener diodesmounted head to tail will be employed. In the present text, one Zenerdiode will therefore also designate two Zener diodes mounted head totail when it will be question of circuits treating alternating current.

What is claimed is:
 1. Lightning arrestor device for protecting aninstallation (3) powered by an electric transmission line (1),comprising at least two assemblies connected in parallel between lineterminals and each assembly comprising a disconnector (P, P′, P₁, P′₁)mounted in series with at least one voltage-dependent variableresistance component, characterized in that the variable resistancecomponent is a Zener diode, at least one assembly comprising at leasttwo Zener diodes (Z₁, Z′₁, Z₂) arranged in parallel.
 2. Lightningarrestor device according to claim 1, characterized in that the at leastone of the assembly comprising at least two Zener diodes arranged inparallel presents electrical links between these diodes and theinstallation which are short links.
 3. Lightning arrestor deviceaccording to claim 1, characterized in that one of the assembliescomprises a Zener diode of which the threshold voltage is greater thanthat of the Zener diodes of the other assemblies.
 4. Lightning arrestordevice according to claim 3, characterized in that the threshold voltageof said assembly is greater than the threshold voltage of the otherassembly by a value (ΔV) equal to substantially the voltage differencebetween the low current value (V_(a)) and high current value (V_(b)) ofthe other assembly.
 5. Device according to claim 1, characterized inthat the threshold voltages of the respective Zener diodes (Z₁, Z′₁) ofsaid assemblies are approximately the same.
 6. Lightning arrestor deviceaccording to claim 5, characterized in that one of the assembliescomprises a Zener diode of which the threshold voltage is greater thanthat of the Zener diodes of the other assemblies.
 7. Lightning arrestordevice according to claim 6, characterized in that the threshold voltageof said assembly is greater than the threshold voltage of the otherassembly by a value (ΔV) equal to substantially the voltage differencebetween the low current value (V_(a)) and high current value (V_(b)) ofthe other assembly.